Ti3SiC2 has been proposed as a mate- rial for ohmic contacts on SiC-based electronic and microelectromechanical devices intended to operate at temperatures >600 °C and/or in corrosive environments. Although SiC is stable and devices made of SiC are capable of functioning at elevated temperatures and in corrosive environments, the lack of stable ohmic contacts has been a barrier to the realization of SiC-based devices. Most metals react with SiC at high temperatures, forming metal silicides and/or carbides. At a minimum, these reactions cause contact resistances to change over time, affecting the performances of devices; in extreme cases, entire contact layers can deteriorate through oxidation, melting, evaporation, or balling up on surfaces.
The proposal to use Ti3SiC2 was made on the basis of a search of the literature on transition-metal carbides, nitrides and borides; metal and compound contacts for SiC; phase stability diagrams; brazing; and methods of deposition. The properties of Ti3SiC2 were found to be exceptionally favorable for making highly stable ohmic contacts on SiC devices for operation at temperatures >600 °C and/or in corrosive environments.
Ti3SiC2 is thermodynamically stable in contact with SiC and is one of the products usually formed in reactions between thin Ti films and 6H SiC substrates after rapid thermal annealing. Ti3SiC2 has electrical conductivity greater than that of Ti or TiC, plus high thermal conductivity and acceptably large resistance to thermal shock. It is rigid, relatively light, and stable at temperatures up to 2,200 °C. Its coefficient of thermal expansion is roughly twice that of SiC; however, like gold, Ti3SiC2 relieves internal stress via motion of dislocations, even at room temperature. Oxidation of Ti3SiC2 follows a parabolic rate law over the temperature range of 900 to 1,400 °C in air, with an activation energy ≈4 eV. The oxidation product is a conformal protective layer of TiO2/SiO2. On the basis of published data, it has been estimated that after 1,000 hours in air at a temperature of 1,100 °F (593 °C) the thickness of the oxide film on Ti3SiC2 would be only ≈11 nm.
Ti3SiC2 can be deposited directly on SiC by sputtering, chemical vapor deposition, or pulsed laser ablation. Because of its thermodynamic stability in contact with SiC, Ti3SiC2 forms a stable interface with a stable electrical resistance. The one major drawback is that Ti3SiC2 reacts with most metals used for electrical interconnections, forming TiC and silicides. Hence, it may be necessary to develop diffusion barriers to enable the use of standard interconnection metals.
This work was done by Harry L. Tuller, Marlene A. Spears, and Richard Mlcak of Boston Microsystems, Inc., for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center,
Commercial Technology Office,
Attn: Steve Fedor,
Mail Stop 4–8,
21000 Brookpark Road,
Cleveland, Ohio 44135.
Refer to LEW-16982.